The organic light emitting diode (OLED) display has attracted attention due to its advantages of low power consumption, high luminance, low cost, wide visual angle and high response speed etc., and has been widely used in the field of organic light emitting technology.
In an OLED display, the current for driving the OLED is determined by the following formula (1-1):Ioled=K(Vgs−Vth)2  (1-1)
where Ioled is current that flows through the OLED, K is a coefficient factor, Vgs is a voltage between the gate and the source of the driving transistor for driving the OLED, and Vth is a threshold voltage of the driving transistor.
Vgs is generally determined by the data signal voltage Vdata (i.e., pixel gray-scale voltage) stored on the hold capacitor Cst and the reference voltage of the hold capacitor Cst. In the prior art, the reference voltage is generally provided by the DC power supply that supplies driving current to the OLED, i.e., being provided by the DC power supply that supplies Vdd or Vss, the reference voltage is equal to the reference voltage Vdd or Vss provided by the DC power supply. Therefore, the current for driving the OLED in the prior art is determined by the following formula (1-2):Ioled=K(Vdata−Vdd−Vth)2  (1-2)
Since Vdd is a voltage signal provided by the DC power supply, all the associated pixels drive the OLED in the whole frame period. The pixel driving current associated with a DC power supply line is relatively large after being converged; and the IR drop on the line is also relatively large. When the voltage Vdd provided by the DC power supply arrives at the reference voltage end on the hold capacitor Cst, the IR drop is ΔR×I, wherein R represents resistance of equivalent layout of the pixel to the power supply, I represents the equivalent current on layout of the power supply, and Δ represents difference between pixels at different positions. The actual reference voltage for charging the hold capacitor Cst is Vdd′(Vdd′=Vdd−ΔR×I).
Since the value of I in ΔR×I is relatively large, R also cannot be reduced infinitely due to process limitation. Therefore, the decreasing amplitude of Vdd relative to Vdd is relatively large. That is, the voltage signal held by the hold capacitor Cst of the pixel would also be influenced by the IR drop, thereby influencing normal display driving.
At present, the difference in the reference voltage caused by different IR drops of pixels at different positions can be compensated by a pixel compensation circuit, however, the circuit is generally complex. A separate line may also be used for providing the reference voltage to the hold capacitor Cst, however, the layout is relatively complex.